I do all agrain now and have had pretty good results so far. I can only boil 3.75 gal, so when I figure a recipe I use the calculator to figure gravities before I brew and top off with the needed water in the fermenter, till now these numbers have been real close. I attemped to make my first lager, a pilsner,the calculator said I should have a 1.064 preboil and I had 1.062. The calculator also said I should have a 1.45 OG....I ended up with a 1.032+. I haven't the foggyest idea what the hell happened. My normal efficiency is 70%, this batch was 65%+ and probably due to using all base malt, and using bottled water due to mine being exceptionaly hard. If my preboil numbers were pretty much on, why then is the OG in the fermentor so far off. After putting wort in fermentor ( a tad over 3 gal) I topped to 5 gal with water and shook the hell outta it, then added yeast and checked OG. Any ideas???

you got 3gals @ 1062. I f you then water this down to 5 gals you get 62x3=186, 186/5=1037. Some loss of extract can be explained by hop retention, careful hop sparging can overcome this or modify your recipe to allow for a certain amount of loss, say 10% or, get yourself a bigger boil capacity and compensate by not having to dilute so much, if you want 5 gals you would be better off boiling 6 gals down. Have you also considered wort temp and subsequent hydrometer reading? Maybe boil over was to blame? Are you using flowers or pellets? flowers retain an awful lot of wort, I normally get a 90% extraction, if i dont rinse my hops i can expect this to drop to approx 80%. You have my heartfelt sympathy, i find loss of extraction extremely irritating.